Chao don't live forever and eventually all Chao will go bakc into their egg cocoon and
disappear. There isn't a way to stop this, and if your Chao doesn't leave behind an
egg, there's no way to "bring your Chao back". However, if your Chao leaves behind
an egg after it cocoons and disappears, you can hatch the eg and get the
reincarnation of your lost Chao. This chao has the same name of the last one and
slightly towards the same preferences of the last one. But other than that, it's a baby
Chao.
Your Chao have to be extremely affectionate with your character in order for it to
reincarnate. Some tips to really boost their affection are...
1. Be sure to keep your Chao well fed, but don't overfeed them.
2. Try to pet thme or cuddel them when they trip. They really appreciate it.
3. (DC Version) Put your Chao in the VMU, or put it in the GBA (GCN Version)
frequently.
4. If a Chao can't swim, they appreciate it when you take them out of the water
when they're stuggling.
5. Never throw or hit your Chao, that alone can kill them if it happens too much.
6. Give your Chao lots of Chaos Drives and Animals.
7. Everyone once in a while cuddle your Chao (petting usually tells them you approve
of their behavior, cuddling just gives the affection).
8. Take your Chao to Kindergarten (more on this later) often.
9. Besides that, just overall be nice to your Chao and they will probably want to stay
forever..